The invention relates to a thread cutting screw to be screwed into a predrilled hole in concrete, masonry, or the like.
Such a thread cutting screw adapted to be screwed into concrete, masonry, or the like without a dowel is known from EP 0 623 759 B 1. The thread geometry in that case is defined by ranges of values for the ratio between outer diameter and core diameter, the ratio between outer diameter and thread pitch, and for the flank angle of the thread. The thread geometry thus is determined by three parameters which may be selected from the ranges claimed so as to obtain screws which will meet the particular requirements in practice.
As is well known, the load bearing capacity of anchorings in concrete can be described by a functional relationship between concrete strength 8w, load bearing thread length or anchoring depth he, and failure load F as defined by the following equation (1)
F=mxc2x7he1.5xc2x7{square root over (xcex2w)}xe2x80x83xe2x80x83(1)
wherein m is a constant having a value of m=13 for uncracked concrete and m=7 for cracked concrete.
Although known thread cutting screws do reach these static loads, they cannot introduce corresponding loads into cracked concrete.
It is the object of the invention to devise a thread forming screw of the type described initially such that high load carrying capacity is achieved even in cracked concrete or masonry. This object is met, in accordance with the invention, where a screw is provided which, when used, permits the transferable loads in concrete or masonry in which cracks, especially dynamic cracks may occur, to reach the level of the load carrying capacity, both in uncracked and cracked concrete.
The invention provides the design engineer with a simple dimensioning aid for the thread configuration and offers the user a simple criterion of choice for selecting the proper screw size depending on the particular load circumstances.
The configuration of the thread of a screw takes into account that, in cracked concrete, only certain pressures can be transmitted between thread flank and concrete. This is based on the finding that the overall load bearing surface AO of the thread flanks in concrete or masonry required for introducing load must be proportional to the static failure load F according to equation (1).
AO=kxe2x80x2*he1.5xe2x80x83xe2x80x83(2)
wherein kxe2x80x2 is a constant.
The value of AO can be determined from the following relationship:                     AO        =                                                            da                -                dbe                            2                        ·                          he              p                        ·                                                            (                                                                                    da                        +                        dbe                                            2                                        ·                    π                                    )                                2                                              +                      p            2                                              (        3        )            
wherein
da=outer thread diameter
dbe=drill edge dimension of the drilling tool for producing the predrilled hole p=pitch.
A relationship between the diameters indicated, the pitch p of the thread, and the load bearing surface AO of the thread flanks can be established by equation (3). If one relates the entire load bearing surface AO to the load bearing thread length he, the result is a relationship between the standardized surface AOn and the load bearing thread length he with
AO/he=AOn=kxe2x80x2{square root over (he)}xe2x80x83xe2x80x83(4)
wherein kxe2x80x2 is a constant and the dimension of the AOn value is given in mm. The thread length he is proportional to the diameter db of the predrilled hole. Equation (4), therefore, can be rewritten as
AOn=k{square root over (db)}xe2x80x83xe2x80x83(5)
wherein k is a constant having a value of 1xe2x89xa6k less than 2.5.
The preferred embodiment of the thread cutting screw according to the invention is one with which the pitch p of the thread fulfills the relationship                     p        ≤                  10          ·                                    da              -              dbe                        2                                              (        6        )            
wherein da=the thread diameter and dbe=the drill edge dimension of a drilling tool for the predrilled hole.
In a practical embodiment it is preferred to select the core diameter dk of the thread by 0.2 to 1 mm smaller than the nominal diameter db of the predrilled hole.
The load bearing surface AO of the thread flanks is a function of the pitch p, the outer thread diameter da, and the drill edge dimension dbe, as indicated above in equation (3). Thus also the standardized surface AOn related to the load bearing thread length he is dependent on these factors p, da, and dbe.
A range of small pitch values is of particular interest for practical use of the invention. For this range, the relationship between the pitch p and the nominal bore diameter db is                     p        ≤                  5          ·                                    db              ·              k                        3                                              (        7        )            
In the range of small pitch values satisfying equation (7), the k value may be selected while making full use of the range from 1xe2x89xa6k less than 2.5 irrespective of the varying outer thread diameters due to manufacturing tolerances.
The dependence of the load bearing thread surface AOn on the nominal bore diameter db depends on the magnitude of the thread flank angle. According to another modification of the invention, however, a thread angle alphaxe2x89xa75xc2x0 is preferred.
If the thread is of unsymmetric design the partial flank angles, each measured with respect to a transverse plane E, have different values. However, the sum of these partial flank angles should also fulfill the relationship of alpha1+alpha2xe2x89xa75xc2x0.
A preferred range of flank angles used with the screw according to the invention, or the sum of the partial flank angles is 30xc2x0  less than alphaxe2x89xa650xc2x0.
A range of from 1.75 less than kxe2x89xa62.0, especially a value of approximately k=1.75 is preferred for the constant k. This limited k range or the k value indicated have proved to be favorable in particular with screws having greater diameters.
Moreover, it is advantageous if the outer thread diameter da is designed to flare conically towards the screw head at a cone angle xcex2 of between 0xc2x0 and 5xc2x0, at least over part of the axial extension of the screw, especially if the setting depths are great because with them the predrilled bores automatically become conical. Such conical flaring may be provided also for the core of the thread, either as an alternative to or together with the conical flaring of the outer thread diameter.
When loaded, the screw experiences its greatest expansion where it exits from the threaded bore. In order to curb the dangerous strains occurring there, particularly the notch stress, another advantageous embodiment of the screw according to the invention is formed with a thread profile which changes over continually from a sharp-edge basic profile to a round thread profile, at least over part of the axial extension of the screw. This round thread profile ends at the location of the greatest expansion of the loaded screw, namely at the transition from the thread to the shank of the screw.